This invention concerns a process for reducing the oxygen content of tantalum for high CV electrolytic capacitors. It involves intimately contacting an alkali metal halide with the tantalum, reacting these substances in a non-oxidizing atmosphere at a maximum temperature of 1200.degree. C. to form and expel the resulting tantalum halide and alkali metal oxide, and then increasing the temperature to 1400.degree. C. for no longer than 10 min to expel any excess alkali metal halide.
For electrolytic capacitors, the oxygen concentration in the tantalum is critical. When the total oxygen content of porous tantalum pellets is above 3000 ppm, capacitors made from such pellets may have unsatisfactory life characteristics. Unfortunately tantalum powder has a great affinity for oxygen, and thus the processing steps which involve heating and subsequent exposure to air inevitably results in an increasing concentration of oxygen. Since the amount of oxygen absorbed will be proportional to the surface area exposed, fine powders with very high CV properties are even more susceptible to the reaction with atmospheric oxygen. The electrical properties of these powders would be improved if the oxygen content were reduced before processing as capacitor anodes.
Alkaline earth metals, aluminum, yttrium, carbon, and tantalum carbide have all been used to deoxygenate tantalum, but there are certain disadvantages to their employment. The alkaline earth metals, aluminum, and yttrium form refractory oxides which must be removed, e.g., by acid leaching, before the material is suitable for capacitors. The amount of carbon must be carefully controlled since residual carbon is also deleterious to capacitors even at levels as low as 50 ppm. Still other methods which have been proposed involve using a thiocyanate treatment or a hydrocarbon or reducing atmosphere during some of the tantalum processing stages to prevent oxidation and thus keep the oxygen content low.